As is well known, dark current in a photoconductive type image pick-up tube comprises a flow of charge carriers within the photoconductive target layer in the absence of light incidence thereon, and is attributed to thermal excitation of carriers in the target material and injection of electrons from the scanning beam into the material. Where PbO is employed as a photoconductive target material, the value of dark current arising from thermal excitation at or about room temperature is almost negligible since PbO has an energy band gap of approximately 2.0 eV. Thus for a PbO target, the actual dark current is principally determined by injection of electrons into the target material from the scanning electron beam.
In this regard, it is known to form a barrier type junction or contact in the target with the intention of obtaining a photoconductive type image pick-up tube having low dark current and lag characteristics. According to this technique, a PbO target is formed with an n-i-p type barrier junction, including a transparent electrically conductive signal electrode principally comprising tin oxide deposited on a glass substrate (which electrode will be referred to hereinafter as a tin oxide layer electrode), a PbO layer adjoining the signal electrode, either one of these layers being rendered n-type conductive, another PbO layer adjoining the first PbO layer and exhibiting substantially intrinsic conductivity, and a p-type layer adjoining the second PbO layer. For details of such a target structure, reference may be had to U.S. Pat. No. 3,444,412.
In such a target structure, the p-type layer precludes injection of electrons from the scanning electron beam into the target and the n-type conductive layer precluding injection of holes from the tin oxide layer electrode into the intrinsically conductive PbO layer, so that only a dark current of limited value may arise. In other words, in this target structure, the surface region of the PbO target to be scanned by an electron beam is made p-type conductive to preclude electron injection into the target from the scanning electron beam. This may be accomplished, for example, by bombarding the target surface with oxygen ions to incorporate an excess quantity of oxygen in the surface region of the target. The surface region of the target formed in this manner, containing a substantially excessive amount of oxygen from a stoichiometric viewpoint, exhibits a distinctly increased conductivity compared with the intrinsic conductivity of the target material. Accordingly, in the p-type conductive region, lateral diffusion of carriers is increased--thus materially impairing the resolution characteristic of an image pick-up tube employing such a target.
With the intention of overcoming the encountered difficulties above-described, a target structure has been proposed in the aforementioned United States patent which has a p-type conductive layer reduced in thickness to diminish the extent of carrier diffusion leakage in such layer. In this case, however the degree of excess oxygen in the p-type conductive layer must be raised as the thickness of such layer is reduced to enable the electron barrier obtained to act effectively and stably for an extended period of time. As a consequence, the conductivity of the p-type conductive layer is not effectively suppressed as desired. This makes it difficult to improve the target resolution characteristic to any desired extent.
In addition, the proposed target structure necessarily suffers the serious disadvantage of being non-homogeneous, since it is technically difficult to form a uniform target region containing excessive oxygen over the entire target area while restricting such region within an extremely thin surface portion of the target. Any local nonuniformity in the p-type conductive layer formed in the state described above will result in a flaw in the output image of an image pick-up tube utilizing such target since at the location the i-p junction must assume a state different from that of the other junction. This obviously constitutes a serious disadvantage for such an image pick-up tube.